Heretofore, an electroless nickel plaing/electroless gold plating process has been employed to mount a board having an independent circuit and electronic parts.
In recent years, due to complication of circuits, it has been desired to inhibit localized corrosion of a nickel plating film. Further, there has been increasing demand for cost reduction of electronic parts or the like due to escalating gold price. In association therewith, technique has attracted attention in which various kinds of electroless palladium plating are carried out between electroless nickel plating and electroless gold plating to reduce thickness of an electroless gold plating film.
For adaptation to such a technique, an electroless palladium-phosphorus alloy plating solution and an electroless pure palladium plating solution have been proposed.
However, in a conventional electroless palladium-phosphorus alloy plating solution, since plating induction time (time that elapses before initiation of plating reaction) is long, deposition rate is low, and in association with complication of a plated circuit, palladium-unplated portions are likely to result, and further, amount of dissolved electroless nickel plating film during palladium plating tends to increase to cause a problem of localized corrosion of nickel.
On the other hand, in an electroless pure palladium plating solution, since a deposited palladium plating film is crystalline, a gold plating film and a palladium plating film are more susceptible to thermal diffusion as compared with an electroless palladium-phosphorus alloy plating film. On account of this, there is a drawback that wire-bonding properties subsequent to soldering of a high melting point solder are poor.